Transactional File locking will not prevent multiple users from editing the same
document, nor give notice that other users are working on the same document.
Multiple users can open and edit a file at the same time and Transactional File
locking does not prevent this.
Rather, it prevents simultaneous file saving.

Note

Transactional file locking is in ownCloud core, and replaces the old
File Locking app. The File Locking app was removed from ownCloud in
version 8.2.1. If your ownCloud server still has the File Locking app, you
must visit your Apps page to verify that it is disabled; the File Locking
app and Transactional File Locking cannot both operate at the same time.

File locking is enabled by default, using the database locking backend.
This places a significant load on your database. Using memcache.locking relieves the database load and improves performance.
Admins of ownCloud servers with heavy workloads should install a memory cache.